1. Field of the Invention
This invention relates to microwave heating apparatus, and more particularly to microwave heating apparatus which heats an article to-be-heated uniformly.
2. Description of the Prior Art
As methods for uniformly heating an article to-be-heated, there have heretofore been ones described below.
(i) Use of a plurality of sources: PA1 (ii) Provision of a plurality of openings for radiation: PA1 (iii) Resonator (secondary antenna): PA1 (iv) Stirrer fan: PA1 (V) Turntable:
In this method, heating apparatus is equipped with a plurality of microwave generating sources. The sources have electric-wave radiation ports for irradiating a heating chamber (oven) by microwaves independently of one another, and the microwaves are radiated towards an article to-be-heated placed within the oven.
Electromagnetic wave energy is supplied by a single power source, and a plurality of electric-wave radiation ports towards a heating chamber are provided.
Electromagnetic waves radiated from a high-frequency power source are excited in an antenna, from which microwaves are radiated towards an article to-be-heated.
Electromagnetic waves in a heating chamber are scattered by rotating a stirrer (electric-wave agitating vane).
A table on which an article to-be-heated is placed is rotated. Thus, the same effect as in scattering electromagnetic waves is achieved.
Among such various methods proposed, especially those (iv) and (v) are structurally simple and greatly effective and are therefore adopted extensively at present. However, they do not bring forth a satisfactory solution to, for example, the problem that the peripheral part of the article to-be-heated or the cap part of a milk bottle is prone to be overheated, i.e., the problem of nonuniform heating.
The nonuniform heating is ascribable to nonuniform distribution of the electromagnetic field in the heating chamber. As the causes therefor, the following are considered:
(a) Inhomogeneous application of radiated waves from a feeding point.
(b) Standing waves due to reflected waves from the wall of the heating chamber.
(c) Standing waves due to reflected waves from the food or the article to-be-heated.
It is a most important object to eliminate especially the causes (a) and (b) among them.
The stirrer fan system (iv) is effective for solving the problem (a). In order to achieve a satisfactory effect, however, the stirrer fan need be made large to the extent of occupying a sufficient space relative to the size of the heating chamber. This leads to the disadvantages that a driving motor becomes large-sized and that the cooking space in the heating chamber becomes small. The turntable system (v) is effective for solving the problem (b). Besides, it is easy in control and comparatively good in the uniform heating. However, it is prone to bring about the nonuniform heating in the radial direction in such a manner that the peripheral part of the article to-be-heated is heated more easily than the central part. Further, by combining it with the multi-source system (i) or the multi-opening radiation system (ii), the nonuniform heating in the radial direction can be reduced. This, however, renders the cost high on account of the increase of the number of power sources of the complication of the structure. It may be said that any practical method for reducing the nonuniform heating has not been developed yet.